Key Points: (1) Co-existing mental health and drug and alcohol problems are very common in clinical practice. (2) The experience of clients with co-existing problems is likened to a huge dysfunctional traffic roundabout. (3) Screening, assessing and intervening with clients with co-existing problems must become core business for health practitioners and health services; skill in this area needs to be a fundamental capability of practitioners working within both specialist mental health and drug and alcohol services. (4) There is no clear indications for a 'best treatment'. (5) There are indications that assessment and brief interventions are useful for some people with co-existing problems. (6) The high prevalence of co-existing problems and evidence for the utility of briefer interventions for some people implies that a 'stepped' model of care may be useful.